caesium
/ˈsiziəm/
noun
- A soft, silvery-white metallic chemical element (symbol Cs) that is highly reactive and used in atomic clocks, drilling fluids, and photoelectric cells.
- Scientists handle caesium with great care because it is highly flammable in air.
- Caesium is so reactive that it can explode when it comes into contact with water.
- The most accurate timekeeping devices, atomic clocks, use the vibrations of caesium atoms.